This invention relates to an indicator which may be attached to the sails of a sailing vessel in order to allow the determination by a sailor that laminar flow of air across the sail's surface is achieved. Laminar airflow creates a negative pressure on the leeward side of the sail and the pressure differential is in turn translated by the hull of the boat into a forward motion.
Various indicators have been developed to allow the determination that the sail is properly trimmed in order to achieve laminar flow of air across the sail. One such apparatus is taught in U.S. Pat. No. 3,763,703 in which a device having air pressure sensors is utilized to measure and compare the pressures on either side of the sail so that the sail may be trimmed to provide a maximum pressure differential between the two sides of the sail. Another system, the subject of U.S. Pat. No. 3,654,807, teaches the use of a pair of temperature sensitive devices, such as thermistors, mounted on either side of the leading edge of an airfoil or sail and which detect the airflow differential resulting from the temperature differences created on either side of the sail by the differential airflow. In U.S. Pat. No. 4,236,409 an indicator for determination of optimum angular positioning of the sail is taught in which a wind vane is interconnected with a computer and angular display whereby the wind direction is determined and the computer determines and then presents to the sailor the proper direction to orient the sail to achieve the optimum angular position of the sail with respect to the wind.
The method of determination of laminar airflow across the sail which has been used for years by sailors is the device known as a "tell-tail". A tell-tail is simply a piece of string or cloth-like material such as nylon of approximately 5-10 inches in length and which is attached to the sail surface. Referring now to FIG. 1, the location of several tell-tails 10 may be seen attached to the sails of the boat in FIG. 1. When the sail is properly positioned with respect to the wind and laminar flow is achieved the tell-tails are caught in the air stream and are pushed directly backward in the direction of the wind as shown in FIG. 2c. The positioning of the tell-tail when non-laminar airflow is achieved is shown in FIGS. 2a and 2b. In these cases, laminar airflow is achieved on one side of the sail only, and the other side is presented with disturbed airflow. In the case of FIG. 2a laminar airflow is achieved on the far side of the sail and the tell-tail for that side, shown in phantom lines, is extending directly backward. On the near side of the sail, the airflow is disturbed in FIG. 2a and the tell-tail is being pushed upward by the turbulent airflow. In FIG. 2b the reverse situation of that illustrated in FIG. 2a is shown with the near side of the sail having laminar airflow as indicated by the tell-tail and the far side having disturbed airflow as indicated by the tell-tail shown in phantom lines. While the disturbed airflow tell-tail is shown as being pushed upwardly in both FIGS. 2a and 2b, the tell-tail can also hang downwardly when non-laminar airflow occurs.
It will be appreciated that in order to determine if laminar airflow is occurring across the sail, it is necessary to be able to see both sides of the sail and to determine whether or not both tell-tails are being forced directly backward as in the case of FIG. 2c. This backward streaming indicates that laminar airflow has been achieved on both sides of the sail. The practical circumstances of sailing do not always allow observation of the tell-tail by a sailor in the cockpit of the boat. For instance, should the sailor be caught in a squall the poor visibility due to the rain will prevent the sailor from observing both the tell-tails. Also, in the case of darkness, it would not be possible for the sailor to see that the tell-tails on either side of the sail were caught in laminar airflow. Finally, as a sailboat size increases, it is not possible for a single individual to be able to visually observe the position of the tell-tails on both sides of the sail in order to determine whether laminar airflow is in fact being achieved. Therefore, a need exists for a simple means of determining the existence of laminar airflow on both sides of a sail or airfoil which can be easily observed by a single individual sailing a boat. In addition a need exists for a device to indicate laminar airflow to a sailor which gives a direct reading of the existence of a laminar flow and which does not depend on extensive instrumentation or complicated measurement devices which may break down under the stringent conditions of the sea or which may be too complicated or cumbersome for a single individual to manipulate during actual sailing.
Also, a need exists for a means for determining laminar airflow across a sail or other airfoil which can be observed by a user in bad weather or at night and which permits the determination of laminar airflow from a distance.